Due to the rising awareness of the global environmental concerns in recent decades, the emission control and fuel consumption have become two key issues in the contemporary automobile industry. By directly delivering the fuel into the engine combustion chamber with accurate quantity and flexible timing, the direct injection (DI) system is one of the most promising technologies to cope with the emission problems without sacrificing the engine performance. Among the available DI systems in the current market, the common rail direct injection (CRDI) system is the most popular one. The injection rate shaping control refers to the method for determining the fuel injection flow rate as a function of time, and is critical for the CRDI system performance.
The current injection rate shaping control can be generally classified into three categories: (i) multiple-injection, (ii) injection pressure variation, and (iii) injection nozzle orifice area variation. The working principle of each category is discussed below. Designs of the present invention shape injection flow rate by controlling an injector needle valve opening and is considered to fall within the methods of category (iii), methods of injection nozzle orifice variation.
Multiple-injection is currently the most common injection rate shaping control method in the CRDI system. The conventional injector is designed to function as an on/off valve, and as such, shapes the rate by a pulse-width-modulation method. The actuation period of the injector control valve is controlled to divide one injection cycle into several discrete injection shots for approximating continuous rate shaping. For a finer approximation, the injection shots per cycle increases. This requires a high bandwidth actuator for the injector, which increases the cost. Moreover, these discrete injection shots induce the pressure vibration in the hydraulic circuit of the CRDI system, which influences the precision of the following injection timing control. This causes inconsistencies of the injection quantity.
In order to solve this problem, several methods have been proposed. An open-loop control method has been developed to establish an injection map by means of injection quantity calibration. By doing a multiple-injection experiment several times, the periodic injection quantity variation can be verified, and the energizing time of the injector actuator can be adjusted to achieve the desired injection quantity. However, as the number of injections per cycle increases, the calibration load becomes extremely heavy. Moreover, even if the injection quantity can be compensated, the injection flow rate will not be the same anymore. Besides the off-line calibration methods, efforts have also been made including feedback control steps. For example, a feedback close-loop control has been developed including an additional pressure sensor provided near an injector upper chamber. This method eliminates the considerable calibration load but still compensates for the injection quantity by adjusting the energizing time.
The injection pressure variation method, as the name implies, controls the fuel injection rate by changing the injection pressure. One way to vary the injection pressure is to apply a pressure intensifier on the injector. Because of the high injection pressure and the extremely short injection window, an additional large power source is required for this system to boost the injection rate. Another way is called a dual common rail system. The pressurized fuel is stored in different reservoirs with different working pressures. In the low-pressure common rail, the pressure is typically around 200-400 bar, and the pressure is about 1200 bar in the high-pressure common rail. A switching valve is provided in order to control the supply fuel pressure from these two common rails to the injector, and then the injection rate changes due to different injection pressures. Besides requiring an additional high pressure reservoir, this method provides only a two-step injection rate shaping.
The injection nozzle orifice area variation method basically controls the injection rate by opening the nozzle orifice area proportionally. In a known system, a rotary valve is controlled by a stepper motor to continuously change the injection nozzle orifice area. The change rate of the nozzle area depends on how fast the stepper motor can respond, which indicates a high requirement for the motor. Another known design is called a two-stage nozzle. Equipped with two groups of injector nozzles and two injector needles, this method realizes a two-step injection rate shaping, but the flexibility of the rate shaping is limited.
Another contemplated method to control the nozzle opening proportion is called direct actuated injector control. The injector actuator as fabricated by specific materials controls the needle lift directly to give proportional needle valve opening. This technology is still under development and also requires a real-time feedback control for the needle displacement. However, the real-time feedback control is difficult to realize because of two barriers. Firstly, due to the lack of adequate sensors, the injector needle position and the injection flow rate are undetectable. Secondly, the CRDI system is nonlinear, which makes the state observer design difficult.